Current non-contact radar sensing technology is generally based on the Doppler radar sensing principle accompanying continuous waves, and implemented in continuous-wave Doppler radar direct conversion receiving way or phase-locked self-injection-locked signal sensing radar transmitting and receiving way. In the former way, a continuous radar wave is used as a detecting wave source. The continuous wave is transmitted to a target object, and a back scattering signal reflected from the target object is analyzed to detect the target object. It is understood that motion of the object will create the Doppler frequency/phasing angle modulation in the back scattering signal. Therefore, by using a direct conversion receiver to remove the continuous wave leaking from the transmitter and the carrier wave of the back scattering signal, the motion of the object can be measured. In the latter way, the wave source of the monitoring system is an oscillating continuous wave generated by an injection phased-locked oscillator. Likewise, the continuous wave is transmitted and propagates to an object, which causes Doppler frequency/phasing angle modulation of a back scattering signal. The back scattering signal is then propagated back into the injection phase-locked oscillator, and a motion of the object can be measured based on the self-injection locked property of the injection phased-locked oscillator.
The above-mentioned non-contact radar sensing techniques, unfortunately, are not suitable directly for signal analysis in the conversion receiver, which simultaneously receives the back scattering signal reflected from the object and the surroundings, because noises resulting from the surroundings are likely to interfere the analysis of the back scattering signals and the identification of the sensed signals.